There are processes, such as the activated sludge process and its modifications, used in waste water treatment where efficient dissolution of gas into liquid is required. In that process, air is disolved into clouded water contaminated with suspended biological mass. To maximize gas dissolution, or transfer, in such a process, a device which releases micron size gas bubbles into the liquid with the least energy requirement and which has a low potential for clogging is desired. Flexible membrane diffuser devices in which gas is forced through micron size pores in an otherwise gas impermeable membrane to bubble upward through a liquid are particularly well suited for this application because, when the device is not in use, the pores through which gas is forced to form bubbles are effectively sealed preventing material which may clog the pores and interfere with efficient diffuser operation from entering the system when gas is not being forced through it. Flexible-membrane devices of the prior art have typically been of round tubular or circular geometry. When such devices have been made in a rectangular plan form with a membrane of uniform porosity and rectangular shape, large stresses which occur in the membrane proximate to its square corners have caused large variations in gas flow rate over the membrane surface resulting in low diffuser efficiency.